Good question. I have read all the mileage threads and haven't come up with an answer. Assuming the engine has no mechanical problem, and the driving is normal, why the big variation?

Nobody drives easier than I do, and so far I struggle to get much over 25mpg (mixed highway/town).

Here's my ideas....

Low tire pressure, tire brand? (I run at 36psi)
Gasoline quality, summer/winter mixture?
Extra high fuel pressure?
Running a slightly rich fuel mixture? (no symptoms of that)
Altitude of where you are driving? (I'm at sea level +600').
Partially clogged air filter (can't see mine since it's enclosed).
Spark plug brand? (mine are OEM)
AC usage? (mine is always on in the summer)

I know my driving habits haven't changed between a bunch of small cars I have owned. This '06 Focus should get better mpg, since I'm aware of the problem, and have been trying everything to get better numbers.

 

> you're on the freeway and use the cruise control for hours at a time. Under these conditions I've never broken low 30s

At what speed? Being a modest HP model, there is a big mileage penalty for that last bit of speed, e.g., expect a big different cruising at 75 MPG vs. 65 MPH, vs. 55 MPH.

 

i dont know about that speed thing, my focus' sweet spot is right around 70. i take a highway to and from work every day. one week ill go 60 the whole way there and get about 28 mpg. the next week ill do 70-75 and pull off 32? i dont know maybe im missing something but its been consistent around those numbers since i got the car

 

Altitude and humidity are the only things I have come up with.

I know that if I travel south I get better mileage vs driving the same path similar style going north. I've proved this to myself on many occasions.

 

This may sound funny, but it's true. In St Louis, the prevailing winds are almost always from the SW. So if you would drive from St Louis toward the west or SW, if you checked the gas mileage after say 300 miles going that direction on a trip, your mileage will be worse that the same route going back home.

How much worse? Depends on your speed, and of course the wind-speed. I'd say a -2mpg would be the minimum loss. AFAIC.... gas mileage checks should be calculated going different directions, to eliminate the "prevailing wind" effect.

 

penguin, I've tried all sorts. 55, 65, 75.. 80...the best I've EVER seen was 34

After 60+ fill-ups I think it's more than enough to cancel out the environmental variables.

With data based on 67 fillups, I have
Mean 27.32mpg
Standard deviation ~2.97mpg

Although I have occasional dips and hikes due to unusual usage (relative to my usual pattern) such as stop and go and long high way trips, the mileage is so consistent my 95% confidence interval is between 26.61 and 28.04mpg

There's only very subtle (few tenth of mpg) between calculating with 2007 and later data or including everything back to 2005.

 

Yeah.....mileage calculations over long periods/many fill-ups are the most accurate. But in the many mileage threads around here, I see a lot of people trying to make mileage conclusions based on single tank-fulls, or even 1/2 tank-fulls using the fuel gauge. Pretty useless....

 

Here's what I have... the black line is what happens when you try to average it by linear regression.